I have used HDR for years and I completely refused to use the built-in HDR when I got my 5D3; I couldn't imagine giving up the control. Then, at some point, I played with it and I liked it. I still save the 3 raw images, but it's nice to have an instant preview of the HDR, and more than once the in-camera HDR shot was good enough to use.

I just wish it didn't make me wait to take another set of pictures. I also wish it was easier to turn on and off (though maybe I can assign it to a shortcut key, i'll have to check).